I must admit I haven't used the 'preview' button myself recently, but I'll give it a try by posting this message......

......And that seems to be working OK.

How are you adding the pictures? Are you trying to post them directly to the forum, or are you doing it by inserting links such as those via Flickr (pasting in BBcode) to embed it in your message?
Direct posting is not possible, and could be that this is why you get the error.

Try and post a bit of text, preview, and then submit. If that works, edit your post and try and add a picture and see where it goes wrong?